1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly to an image forming apparatus including a fixing unit having a heating roller and a pressing roller to fix a toner image transferred to a sheet, in which the pressing roller is released from the heating roller when the heating roller is overheated to prevent the heating roller and the pressing roller from being fused to each other.
2. Description of the Related Art
In general, as an example of an image forming apparatus, an electro-photographic image forming apparatus includes a charging unit, a laser scanning unit as an exposing unit, a developing unit, a transferring unit, and a fixing unit around a photoconductive drum. The surface of the photoconductive drum charged by the charging unit is exposed by the laser scanning unit to form an electrostatic latent image. The developing unit develops the electrostatic latent image to a toner image and the toner image is transferred to a sheet by the transferring unit. The toner image transferred to the sheet is fixed by the fixing unit to be transmitted to the outside.
In the above-described image forming apparatus, the transferred toner image is fixed by simultaneously applying heat and pressure to a sheet.
As illustrated in FIG. 1, the fixing unit includes a heating roller 10 rotatably provided to fuse a toner image 41 to a sheet 40 by heat, a pressing roller 20 rotatably provided to face the heating roller 10 and pressing the sheet 40 toward the heating roller 10, and pressing units 30 and 31 supporting the rotating shaft 21 of the pressing roller 20 so that the pressing roller 20 can press the sheet 40 to generate pressure.
A heat lamp 11 is provided in a center of the heating roller 10 so that the heating roller 10 is heated by radiant heat from the heat lamp 11. A thermistor 50 making contact with an outer peripheral portion of the heating roller 10 so as to sense a surface temperature of the heating roller 10 and a thermostat 60 shutting off a power being supplied to the heat lamp 11 when the temperature of the heating roller 10 is higher than a critical temperature are provided on one side of the heating roller 10. The thermistor 50 senses the surface temperature of the heating roller 10 to transmit the sensed temperature to a microcomputer of the image forming apparatus. The microcomputer controls the power supplied to the heat lamp 11 in accordance with the sensed surface temperature of the heating roller 10 to maintain the surface temperature of the heating roller 10 to be in a required uniform range. Also, an internal terminal of the thermostat 60 is opened when the temperature of the heating roller 10 is higher than the critical temperature to intercept the power source supplied to the heat lamp 11.
In general, in the image forming apparatus, when the heat lamp 11 continuously generates heat to overheat the heating roller 10, the pressing roller 20 is melted due to a high temperature so that the pressing roller 20 is fusion-welded to the heating roller 10.
Therefore, a conventional pressure releasing apparatus is provided in order to release the pressing roller 20 from the heating roller 10. The microcomputer of the image forming apparatus senses the temperature of the heating roller 10 by the thermistor 50 and, when it is determined that the heating roller 10 is overheated, and operates the pressure releasing apparatus to release the pressure of the pressing roller by the pressing unit 30 and 31 and to prevent the heating roller 10 and the pressing roller 20 from being fused to each other. However, the pressing roller 20 cannot be released and separated from the heating roller 10 enough to have a gap between the pressing roller 20 and the heating roller 10 or a recording medium disposed between the pressing roller 20 and the heating roller 10.
In general, the microcomputer of the image forming apparatus analog-to-digital (A/D) converts the voltage value sensed by the thermistor 50 by an A/D port to sense a temperature value. However, the A/D port of the microcomputer may be damaged by the static electricity of the sheet 40 or the surge voltage of a power source line. In such a case, the microcomputer erroneously operates.
When the heating roller 20 is continuously heated in a state in which the pressing roller 20 is not released because the pressure releasing apparatus does not operate due to the malfunction of the microcomputer, the heating roller 10 and the pressing roller 20 are fusion-welded welded to each other to break the thermostat 60.